Process for forming sulfide layers by photochemical vapor deposition

ABSTRACT

The specification discloses a low-temperature process for depositing a layer of a sulfide of a chosen element, such as zinc sulfide, on the surface of a substrate while simultaneously avoiding damage to the substrate. The process comprises exposing the substrate to a selected vapor phase reactant containing the chosen metal, such as dimethyl zinc, in the presence of neutral, charge-free sulfur atoms formed in a manner which avoids the generation of charged particles and high energy radiation that would damage the substrate. The sulfur atoms react with the vapor phase reactant to form the sulfide thereof, such as zinc sulfide, which deposits as a layer on the surface of the substrate. In a preferred process embodiment, the neutral sulfur atoms are generated by photochemical dissociation. In addition, there is disclosed a process for forming a native sulfide layer on the surface of a chosen substrate by exposing the substrate to neutral, charge-free sulfur atoms.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the fabrication of semiconductordevices and circuits in which a sulfide layer is formed on the surfaceof a selected substrate and, more particularly, to the deposition of asulfide layer or the formation of a native sulfide layer on a substrateat a low temperature and without causing charge damage or radiationdamage to the substrate.

2. Description of the Prior Art

In the manufacture of semiconductor devices and circuits, it is oftennecessary to form an insulator or dielectric layer on the surface of asemiconductor substrate to provide electrical insulation betweenadjacent layers or structures. The physical and electrical properties ofthe dielectric layer are important in determining the electricalperformance of the completed device or circuit. Frequently useddielectric materials include silicon dioxide, glasses of silicon dioxideand other oxides, silicon nitride, aluminum oxide, and organic films,such as polyimide or Teflon, as discussed, for example, by J. A. Amick,G. L. Schnable, and J. L. Vossen, in the publication entitled"Deposition techniques for dielectric films on semiconductor devices",in the Journal of Vacuum Science and Technology, Vol. 14, No. 5,Sept./Oct. 1977, at page 1053. In addition to these latter materials,sulfides have more recently been used to provide thin film passivationlayers which are transmissive to radiation in the infrared range, foruse in such devices as infrared radiation detectors.

The conventional methods for forming sulfide thin films involve eithersputtering or evaporation processes. In accordance with a knownnon-reactive sputtering process, as described, for example, by B. R.Critchley and P. R. C. Stevens, in the Journal of Physics D, AppliedPhysics, Vol. 11, 1978, pages 491 to 498, a disk of a selected sulfidematerial, such as zinc sulfide (ZnS), is bombarded in a reaction chamberwith inert ions such as argon ions. The bombarding inert ions cause theZnS to vaporize from the target (disk), and the vaporized ZnSsubsequently deposits on the selected substrate. In such a sputteringprocess, the bombarding ions are formed by subjecting the chosenbombarding material, such as argon, to a radio frequency (rf) or directcurrent (dc) discharge. However, as a result of the exposure of thechosen bombarding material to the discharge, numerous extraneous ionizedand neutral particles and high energy radiation with wavelengths as lowas 500 angstroms or lower are produced. These extraneous particles thenbombard the surface of the substrate on which the sulfide is beingformed and cause damage thereto by altering the quantity anddistribution of charge therein. In addition, the bombardment of thesubstrate surface by these particles causes the formation of additionalcharged particles and radiation, which may also damage the substrate.This alteration in the charge of the substrate undesirably alters theelectrical performance of the substrate and any structures formedtherein. In addition, the deposited sulfide may incorporate charges ordangling bonds, which create high surface state densities at theinterface between the semiconductor substrate and the deposited sulfide,and which will trap charges when a voltage is applied to the device,thereby preventing optimum device performance. The damage produced bycharge bombardment and radiation bombardment is particularly noticeablewhen the substrate comprises an electrically sensitive device, such as acharge coupled device or a device formed of certain compoundsemiconductor materials, such as mercury cadmium telluride, indiumantimonide, or gallium arsenide.

In accordance with a known evaporation process to form a thin sulfidelayer, as described, for example, by K. Pulker and J. Maser, in ThinSolid Films, Vol. 59, 1979, pages 65 to 76, a source comprising theselected sulfide, such as arsenic sulfide, is placed in a reactionchamber and is raised to an elevated temperature sufficient to causeevaporation of the sulfide, which subsequently deposits on the selectedsubstrate. However, the sulfide films formed by evaporation processesgenerally have non-uniform surface morphology and non-reproducibledeviations from stoichiometric composition, which degrade the electricalperformance and reliability of the device on which the sulfide layer isformed.

It is the alleviation of the prior art problem of imparting damage tosensitive devices due to charge bombardment and radiation bombardmentduring the formation of a good quality sulfide layer thereon to whichthe present invention is directed.

SUMMARY OF THE INVENTION

The general purpose of this invention is to provide a new and improvedprocess for depositing a layer of a sulfide of a chosen element on thesurface of a selected substrate while simultaneously avoiding substratedamage due to charged particles or broadband electromagnetic radiation.This process possesses most, if not all, of the advantages of the aboveprior art sulfide deposition processes, while overcoming theirabove-mentioned significant disadvantages.

The above general purpose of this invention is accomplished by exposingthe substrate to a selected vapor phase reactant in the presence ofneutral, charge-free sulfur atoms. The sulfur atoms react with the vaporphase reactant to form the desired sulfide, which deposits as a layer onthe substrate. The use of neutral, charge-free sulfur atoms avoidsdamage to the substrate due to charge bombardment or radiationbombardment.

Accordingly, it is a further purpose of the present invention to providea new and improved process for forming a layer of a sulfide of a chosenelement on the surface of a selected substrate, wherein the sulfidelayer has good insulating properties.

Another purpose of the present invention is to provide a process of thetype described which minimizes the value of the surface state density atthe sulfide/semiconductor substrate interface and thus minimizes thecharge traps in the deposited sulfide layer.

Still another purpose is to provide a process of the type describedwhich produces a low density of generation/recombination centers at theinterface between the deposited sulfide layer and the substrate, andthus provides good minority carrier lifetime in the substrate andreduced susceptibility to radiation damage in the device formed by thisprocess.

Another purpose is to provide a process of the type described in whichthe radiation hardness of the substrate can be maintained duringdeposition of the sulfide layer thereon.

Yet another purpose is to provide a process of the type described forforming a layer of a sulfide material having desirable opticallyantireflective properties.

A further purpose is to provide a process of the type described in whichthe temperature is sufficiently low so as to avoid thermal damage to atemperature-sensitive substrate.

Another purpose is to provide a process of the type described which isperformed at a temperature as low as room temperature (e.g., 30° C.) andthus eliminates the problems of boundary migration and the resultingdecreased device yields.

Yet another purpose of the present invention is to provide a process ofthe type described which produces dense, non-granular, adherent sulfidefilms of stoichiometric composition on selected substrates.

Another purpose is to provide a process of the type described whichproduces a high quality sulfide material reproducibly and with highyield.

It is a further purpose of the present invention to provide a new andimproved process for forming a layer of a native sulfide on the surfaceof a selected substrate.

Another purpose of the present invention is to provide a new andimproved process for depositing a layer of a chosen sulfide containing aselected dopant material on the surface of a substrate while avoidingdamage to the substrate due to charge bombardment or radiationbombardment.

A feature of the present invention is that a low-temperaturephotochemical vapor deposition process may be used to form theabove-described deposited sulfide layer or native sulfide layer.

The foregoing and other advantages and features of the present inventionwill become more readily apparent from the following more particulardescription of the preferred embodiments of the invention, asillustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates, in schematic form, a preferred apparatus whichmay be adapted for carrying out the processes according to variousembodiments of the present invention, in which neutral sulfur atoms areformed by either the mercury photosensitized dissociation or the directdissociation of a sulfur-containing precursor, and are reacted to formeither an undoped or a doped sulfide layer.

DETAILED DESCRIPTION OF THE INVENTION

The FIGURE shows, in simplified form, an apparatus suitable forimplementation of two process embodiments of the present invention inwhich neutral sulfur atoms are formed by the mercury photosensitizeddissociation of a chemically unreactive sulfur-containing precursor,such as carbonyl sulfide, hydrogen sulfide, dimethyl sulfide, carbondisulfide, and methyl mercaptan. (The term "chemically unreactive" isused herein to denote that a substance will not react with thedesignated reactants under normal mixture conditions.) A reactionchamber 10, in which the chemical vapor deposition reaction occurs, isprovided with a quartz window 12, which is integral with the top surfaceof the reaction chamber 10. The quartz window 12 is transmissive to theselected wavelength of radiation used to initiate the desiredphotochemical reaction to be discussed below. This radiation 14 of aselected wavelength is produced by the radiation-producing means 16,which may be, for example, an array of low pressure mercury vapor arclamps. Within the reaction chamber 10, there is a substrate holder 18,which receives a substrate 20 onto which the desired sulfide layer is tobe deposited. The substrate 20 may be a semiconductor material havingelectronic structures, such as regions of predetermined conductivity,formed therein. In addition, the substrate 20 may comprise asemiconductor material having a layer of a second material, such as anative sulfide, formed on the surface thereof, such that the sulfidelayer of the present invention is deposited on the surface of thissecond material. Alternatively, the substrate 20 may comprise anelectrooptical component or element, such as a lens or mirror, or a bodyof a chosen material, such as a plastic material, on which the sulfideis deposited.

External to the reaction chamber 10 and adjacent to the bottom surfacethereof, there are heating elements 21, which may be formed, forexample, of Nichrome wire and are activated by applying a controlledvoltage thereto. The heating elements 21 may be used optionally to heatthe substrate 20 to the required temperature so that appropriateproperties of the deposited layer, such as density, may be obtained. Thetemperature in the chamber 10 may be maintained as low as roomtemperature (e.g., 30° C.) or as high as required (e.g., 300° C. orhigher). However, since mercury vapor arc lamps, for example, becomeless efficient at increased temperatures, it is necessary to provideexternal water cooling or an external air or nitrogen cooling source tocool these lamps and remove radiant heat produced by the substrate andsubstrate holder 18 at certain elevated temperatures. For this purpose,the radiation-producing means 16 is completely contained within theenclosure 23, which may be formed of aluminum, and an external coolingmeans 25, such as pipes with water flowing therethrough as shown in theFIGURE or flowing nitrogen gas, is activated to apply cooling to theenclosure 23. The enclosure 23 is secured to the outside surface of thereaction chamber 10 which surrounds the quartz window 12, but may beremoved therefrom as required. Thus, the processing temperature ismaintained at a level such that sufficient cooling of the mercury lampscan be accomplished in order to provide efficient lamp performance. Theenclosure 23 also provides eye protection to the operator from theradiation 14. Leading from the reaction chamber 10, there is a tube 22which passes through a valve 24 and then to a vacuum-producing means,such as a pump (not shown), which is used to evacuate the chamber 10 toa sufficiently low pressure to allow the vapor deposition reaction tooccur.

External to the reaction chamber 10, there are the chambers 26 and 28which contain the individual reactant gases for the selected chemicalvapor deposition reaction, for example, dimethyl zinc and carbonylsulfide. The chambers 26 and 28 are connected to the control valves orflowmeters 30 and 32, respectively, which are used to control theamounts of reactants which are introduced into a tube 34. Alternatively,for second and fourth process embodiments of the present invention(discussed below), there are included a third chamber 27, which containsthe precursor of the selected dopant material, such as trimethyl indium,and a corresponding control valve or flowmeter 31, which controls theamount of dopant precursor introduced from the chamber 27 into the tube34, where it is mixed with the other reactant gases discussed above. Inaddition, there may be included a fourth chamber and associated controlvalve (not shown) for containing an inert carrier gas, such as nitrogen.The carrier gas is introduced into the chamber, such as chamber 26,containing the selected vapor phase reactant, such as dimethyl zinc,which has a low vapor pressure, and serves to carry the vapor phasereactant out of the chamber 26. In order to simplify the FIGURE herein,the carrier gas mechanism is not shown since the use of carrier gas iswell known in the art.

The reactant gases flow through the tube 34 into a chamber 36 whichcontains a pool of mercury (at room temperature) having mercury vaporabove it, at a vapor pressure of 10⁻³ Torr at 30° C. The reactant gasesthus become mixed with mercury vapor in the chamber 36 and this reactantgas mixture then passes through a tube 38 and into the reaction chamber10, where the chemical vapor deposition reaction may be brought about.The components of the apparatus shown in the FIGURE may be constructedof stainless steel or aluminum, unless otherwise specified. Theapparatus shown in the FIGURE may be used for either a low pressurecontinuous flow photochemical reactor system, in which there is acontinuous influx of reactant gases and removal of reaction by-productsduring the photochemical reaction process, or for a static photochemicalreactor system, in which specified amounts of reactants are introducedinto the reaction chamber, the flow of reactant gases is stopped, andthen the photochemical reaction process is allowed to occur.

In practicing the present invention in accordance with the firstembodiment thereof, which depends on the mercury-sensitizedphotochemical generation of atomic sulfur, and using the apparatus ofthe FIGURE with only two reactant gas chambers (e.g., the chambers 26and 28), a chemical vapor deposition process is performed as generallydescribed by Werner Kern and Richard S. Rosler in the publicationentitled, "Advances in Deposition Processes for Passivation Films", inthe Journal of Vacuum Science and Technology, Vol 14, No. 5, Sept.-Oct.1977, pages 1082 to 1099, in the discussion of low pressure chemicalvapor deposition processes, and further discussed in the book entitled"Photochemistry" by J. G. Calvert and J. N. Pitts, Jr., John Wiley andSons, Inc. New York, 1966, both references being incorporated herein byreference. The reaction chamber 10 is evacuated by the vacuum-producingmeans to a predetermined pressure, for example 1 to 4 torr (millimetersof mercury). (This operating pressure is selected to maximize the meanfree path and lifetime of the reactants in order to produce a sulfidewith good morphology and good step coverage, while at the same timeproducing practical deposition rates.) The selected vapor phasereactant, such as dimethyl zinc [Zn(CH₃)₂ ], is placed in a chamber suchas the chamber 26, and a chemically unreactive sulfur-containingprecursor, such as carbonyl sulfide (COS), is placed in a chamber suchas the chamber 28. The valves 30 and 32 are set so that the reactantsfrom the chambers 26 and 28, respectively, in a predetermined ratio andat a predetermined flow rate may pass into the tube 34 and then into thechamber 36, which contains a pool of mercury. These reactant gasesbecome mixed with mercury vapor in the chamber 36 and pass from thechamber 36 through the tube 38 into the reaction chamber 10, which ismaintained at approximately room temperature (e.g., 30° C.) or up to200° C. or higher. The reaction temperature is chosen to maximize thequality of the deposited sulfide, while at the same time minimizingthermal damage to the substrate, as discussed in further detail below.The radiation-producing means 16 is activated and produces the radiationof a selected wavelength required to produce the desired photochemicalreaction (e.g., 2537 Å which is the resonance absorption line to producemercury in an excited state). The radiation 14 passes through the quartzwindow 12 into the reaction chamber 10, where it excites the mercury(Hg) atoms in the reactant gas mixture to form mercury atoms in anexcited state (Hg*), which is approximately 5 electron volts abovenormal ground state, but non-ionized, as shown in Equation (1) below.The Hg* then collides with the sulfur-containing precursor, such as COS,transferring energy thereto and causes the precursor to dissociate andproduce atomic sulfur (S), as shown in Equation (2a) below. In addition,the Hg* may react with the selected vapor phase reactant, such asZn(CH₃)₂ to produce a charge-free reactive radical, such as a methylzinc radical, as shown in Equation (2b) below.

Finally, the atomic sulfur reacts with the reactant, Zn(CH₃)₂ or thereactive radical thereof, to produce the desired sulfide, such as zincsulfide (ZnS) as shown in Equations (3a) and (3b) below. The reactantgas ratio is controlled in order to control the stoichiometriccomposition of the sulphide product, as discussed in further detail inExample 1. The by-products of the reactions of Equations (3a) and (3b)comprise hydrocarbon volatiles, such as ethane, dimethyl sulfide, andhigher alkanes.

    Hg+hν(2537 Å)→Hg*                            (1)

where

h=Planck's constant

ν=frequency of absorbed radiation

    Hg*+COS→S+CO+Hg                                     (2a)

    Zn(CH.sub.3).sub.2 +Hg*→·Zn(CH.sub.3)+·CH.sub.3 +Hg                                                       (2b)

    Zn(CH.sub.3).sub.2 +S→ZnS+By-products               (3a)

    ·Zn(CH.sub.3)+S→ZnS+By-products            (3b)

The reaction of Hg* with Zn(CH₃)₂ to form the radicals thereof as shownin Equation (2b) is a side reaction which is not necessary to theprocess of the present invention, but leads to the same end product asthe major reaction path shown in Equations (1), (2a), and (3a). Asindicated previously, the atomic sulfur produced in Equation (2a) abovemay react directly with the selected vapor phase reactant as describedin Equation (3a) above, without first forming an intermediate radical.

Alternatively, the atomic sulfur required for this first processembodiment of the present invention may be formed by themercury-sensitized dissociation of other compounds containing sulfur,such as hydrogen sulfide (H₂ S), dimethyl sulfide [S(CH₃)₂ ] or otherdialkyl sulfides, carbon disulfide (CS₂), methyl mercaptan [HS(CH₃)] andother photo-dissociable sulfur-containing compounds having sufficientvapor pressure to go into the gas phase (e.g. having a vapor pressure of1 to 3 torr at room temperature). It is noted that uponphoto-dissociation of some of the above-noted sulfur-containingcompounds, there may also be produced a certain amount ofsulfur-containing molecular fragments, rather than atomic sulfur. Whilesuch sulfur-containing molecular fragments may contribute to theformation of a sulfide layer, the quality of the sulfide layer soproduced may be less desirable than the sulfide layer formed from atomicsulfur. The preferred process embodiment of the present inventioncomprises the formation of discrete atomic sulfur which reacts with thevapor phase reactant in a controllable manner and without undesired sidereactions.

The selected vapor phase reactant used in the process of the presentinvention comprises a volatile compound containing the metal or cationwhose sulfide is being formed and which is capable of reaction whichsulfur atoms as described herein. Some suitable metal-containingcontaining vapor phase reactants are a methyl compound or other alkyl oralkoxy compound, a chloride compound or other halide compound, or ahydrogen compound. For example, zinc sulfide is deposited by the processof the present invention using a dimethyl zinc reactant, as previouslydiscussed, or a zinc chloride (ZnCl₂) reactant. Lead sulfide (PbS) isdeposited using a tetramethyl lead [Pb(CH₃)₄ ] reactant; arsenictrisulfide (As₂ S₃) is deposited using an arsine (AsH₃) reactant; indiumtrisulfide (In₂ S₃) is deposited using a trimethyl indium [In(CH₃)₃)]reactant; and cadmium sulfide is deposited using a dimethyl cadmium[Cd(CH₃)₂ ] reactant.

By the above-described process of the present invention, sulfur atomsare produced by a photochemical dissociation process which generatesonly neutral, charge-free particles. The term "sulfur atom" or "atomicsulfur" is used herein to designate a sulfur atom which is a neutralspecies having unbonded electrons in its outer electron shell. Thepresence of these unbonded electrons causes the sulfur atom to be highlyreactive, to try to gain two more electrons and form a stable, completedouter electron shell. In addition to being neutral (non-ionized) andcharge-free, the atomic sulfur used in the present invention is formedin a benign manner which avoids the generation of charged particles orhigh energy radiation that may damage the substrate or the interfacebetween the substrate and the deposited sulfide. Thus, the process ofthe present invention is charge-free since it is an electrically neutralprocess which generates no positive or negative particles or ions, orfree electrons. Consequently, the process of the present inventionavoids the previously discussed prior art problem of substrate damagedue to bombardment by charged particles or high energy radiation. It isintended to include within the scope of the present invention not onlyphotochemically generated neutral atomic sulfur but also any neutral,charge-free sulfur atoms formed in a manner which avoids the generationof charged particles or high energy radiation.

More specifically, by the process of the present invention, it isanticipated that the value of the surface state density at thesulfide/semiconductor substrate interface and the charge traps in thesulfide or insulator layer will be minimized. In addition, a low densityof generation/recombination centers (i.e., dangling bonds or traps) isexpected at the interface between the deposited sulfide and thesubstrate, and thus good minority carrier lifetime in the substrate isexpected in devices formed by the process of the present invention.

The problem of substrate damage due to charge-bombardment orradiation-bombardment is particularly important when processingradiation-hardened devices (i.e. devices which are required to beresistant to damage by gamma radiation, such as for use in spaceapplications). When certain solid materials are subjected to gammaradiation, electrons are ejected from their normal position and becometrapped in the various structural defects of the crystal lattice ornetwork of the material. This latter effect alters the electricalproperties of the material such that device performance is degraded. Bythe very nature of some prior art processes for sulfide depositionpreviously discussed, dangling bonds or charge traps are created in thesubstrate or at the interface between the deposited sulfide and thesubstrate due to charge-bombardment or radiation-bombardment of thesubstrate. These dangling bonds or traps normally increase theprobability of radiation damage to the device since they provide sitesfor entrapment of charged species. As discussed above, the process ofthe present invention is charge-free and the formation of charge trapsin the sulfide or at the sulfide/substrate interface is minimized.Consequently, the process of the present invention is capable ofmaintaining the radiation-hardness of a device during sulfidedeposition. Moreover, it is known that at increased temperatures,structural defects are more likely to be produced in a device beingprocessed, which, in turn, would increase the susceptibility of thedevice to radiation damage. The process of the present inventionovercomes this latter problem by using a low processing temperature,such as 30° to 300° C. Thus, the process of the present inventionmaintains the radiation hardness of a given device both because of thecharge-free nature of the process and because of the low processingtemperature. In addition, because of the low processing temperature ofthe present invention, this process is especially well suited forforming a sulfide layer on a temperature-sensitive compoundsemiconductor material (such as mercury cadmium telluride, indiumantimonide, or gallium arsenide) or on a temperature-sensitive plasticmaterial (such as a polycarbonate, an acrylic, or a polyimide).

Moreover, the problem of boundary migration has been eliminated sincethe process of the present invention can be conducted at a relativelylow temperature, i.e. as low as room temperature; and the associatedproblem of decreased device yield encountered in the high temperaturefabrication of certain semiconductor devices has been avoided.

Further, the process of the present invention is highly reproducible,reliable, and capable of a high degree of control over the sulfidegrowth process by, among other things, controlling the initiatingradiation for the photochemical reaction. Finally, the sulfide layerformed by the process of the present invention has excellentstoichiometry, morphological characteristics and infrared transmissioncharacteristics, and is dense, non-granular, durable, and stronglyadherent to a variety of substrate surfaces, as discussed in detail inExample 1 herein.

The sulfide layers formed by the process of the present invention areuseful as a passivation layer or as an antireflective coating which istransparent to radiation in the infrared range, particularly ontemperature-sensitive substrates. More specifically, a zinc sulfide filmformed in accordance with the present invention is a dielectric materialwhich is useful for providing passivation of devices formed fromcompound semiconductor materials, such as gallium arsenide and mercurycadmium telluride. Arsenic trisulfide is useful for the passivation oflead sulfide infrared detectors. Lead sulfide, which is a semiconductormaterial, is useful as the active element in infrared detectors andinfrared charge-coupled devices. Moreover, the sulfides of the presentinvention may be applied to the surface of an optical element orcomponent (such as a glass or plastic lens or mirror) as anantireflective or enhanced reflective coating. For such applications asdiscussed above, the sulfide is usually provided as a thin layer (e.g.,0.1 to 10 micrometers).

In addition, the sulfide layers formed by the process of the presentinvention may be provided as a patterned layer. In such a case, thesulfide material may be deposited as a continuous layer and subsequentlypreferentially etched in a predetermined pattern using photolithographicprocedures which are known in the art. Alternatively, the sulfidematerial may be deposited in a predetermined pattern using a mask at thesubstrate surface or by using focused initiating radiation which strikesonly selected portions of the substrate, as described in U.S. Pat. No.4,226,932, assigned to the present assignee.

Furthermore, in accordance with a second process embodiment of thepresent invention, there may be deposited a sulfide layer comprising asulfide material, such as zinc sulfide, having a chosen dopant, such astin, incorporated therein to modulate the refractive index of the hostsulfide material as may be required for a particular opticalapplication. This second process embodiment of the present invention maybe performed by practicing the present invention as described above,using the apparatus of the FIGURE with three reactant gas chambers(e.g., the chambers 26, 27, and 28). The selected vapor phase reactant,such as Zn(CH₃)₂, is exposed to simultaneously formed andphotochemically generated neutral sulfur atoms and neutral atoms ormolecular fragments of the selected dopant, such as tin-containingradicals. The neutral reactant particles are produced by the mercuryphotosensitized dissociation of a chemically unreactivesulfur-containing precursor, such as carbonyl sulfide (COS) and achemically unreactive dopant-containing precursor, such as tetramethyltin [Sn(CH₃)₄ ], which results in the generation of atomic sulfur andneutral trimethyl tin radicals, respectively, as shown in Equations (4)through (6) below. The atomic sulfur and trimethyl tin radicalssubsequently react with the Zn(CH₃)₂ to form tin-doped zine sulfide(Sn:ZnS) as shown in Equation (7) below. The by-products noted inEquation (7) comprise hydrocarbon volatiles, as previously discussed.

    Hg+hν(2537 Å)→Hg*                            (4)

    Hg*+COS→S+CO+Hg                                     (5)

    Hg*+Sn(CH.sub.3).sub.4 →·Sn(CH.sub.3).sub.3 +·CH.sub.3 +Hg (6)

    Zn(CH.sub.3).sub.2 +·Sn(CH.sub.3).sub.3 +S→Sn:ZnS+By-products (7)

A mechanistic alternative to the sequence described above involves thesimultaneous reaction of both Zn(CH₃)₂ and Sn(CH₃)₄ with atomic sulfur,as indicated in Equations (8) through (10) below. Thus, atomic sulfuralone may be sufficient to bring about the desired reaction as shown inEquation (10) below. Consequently, the production of neutral particlesof the selected dopant material as described above in Equation (6) maynot be necessary in practicing the second process embodiment of thisinvention, but may occur.

    Hg+hν(2537 Å)→Hg*                            (8)

    Hg*+COS→S+CO+Hg                                     (9)

    Zn(CH.sub.3).sub.2 +Sn(CH.sub.3).sub.4 +S→Sn:ZnS+By-products. (10)

The procedure followed to accomplish this second process embodiment ofthe invention is essentially as described above with respect to thefirst process embodiment of the present invention, except thatadditionally, a controlled amount of a selected dopant-containingprecursor is introduced from a chamber such as the chamber 27 throughthe control valve 31 into the tube 34, where it mixes with the reactantgases from the chambers 26 and 28.

Thus, by this second process embodiment of the present invention, adoped sulfide film may be deposited by a low-temperature process whichavoids the generation of charged particles and high energy radiation andtheir bombardment damage to the substrate. Additionally, this secondprocess embodiment has all the advantages enumerated above with respectto the formation of an undoped sulfide layer by the first processembodiment of the present invention.

In accordance with this second process embodiment of the presentinvention, other dopants besides tin may be incorporated in thedeposited sulfide layer by addition of the correspondingdopant-containing precursor to the reactant gas mixture. For example,trimethyl indium [In(CH₃)₃ ]may be used for indium doping, diborane (B₂H₆) may be used for boron doping, arsine (AsH₃) may be used for arsenicdoping, phosphine (PH₃) may be used for phosphorus doping, hydrogenselenide (H₂ Se) may be used for selenium doping, or hydrogen telluride(H₂ Te) may be used for tellurium doping. Other dopant-containingprecursors which are capable of the mercury photosensitized dissociationreaction of the type discussed herein may also be used. In addition,other sulfide materials besides ZnS, which have been discussed withrespect to the first process embodiment of the present invention, may beformed as a doped sulfide as described herein. For example, indium-dopedarsenic trisulfide may be formed in accordance with the presentinvention using trimethyl indium and trimethyl arsenic in predeterminedratios as the chosen reactants.

In accordance with the third process embodiment of the presentinvention, the required neutral sulfur atoms are formed by a directphotochemical dissociation reaction of a sulfur-containing precursor,thus eliminating the need for mercury photosensitization. The apparatusshown in the figure is used except that the chamber 36 which holds themercury is omitted and only two reactant gas chambers (e.g. chamber 26and 28) are used.

In practicing the present invention in accordance with the third processembodiment thereof and using the apparatus shown in the FIGURE, omittingthe chamber 36 and using only the chambers 26 and 28, the generalprocess described in relation to the first process embodiment of thepresent invention is followed, except that no mercury is used forphotosensitization. The valves 30 and 32 are set so that the reactantgases, such as Zn(CH₃)₂ and COS, from the chambers 26 and 28,respectively, pass in a predetermined ratio and at a predetermined flowrate into the tube 34 and then into the reaction chamber 10. Theradiation-producing means 16 is activated and produces the radiation 14of a selected wavelength, which is the appropriate wavelength to causethe direct dissociation of the selected sulfur-containing precursor(e.g., 1750-1950 Å for COS). The radiation 14 passes through the window12, which is formed of a material that is transparent to the wavelengthof the radiation 14. The radiation 14 passes into the reaction chamber10, where it causes the dissociation of the selected sulfur-containingprecursor, such as COS, into atomic sulfur, which then reacts with theselected vapor phase reactant, such as Zn(CH₃)₂, to form the desiredsulfide, such as ZnS, as shown in Equations (11) and (12) below. Theby-products noted in Equation (12) comprise hydrocarbon volatiles, aspreviously discussed.

    COS+hν(1849 Å)→S+CO                          (11)

    Zn(CH.sub.3).sub.2 +S→ZnS+By-products               (12)

Alternatively, the atomic sulfur required for this third processembodiment of the present invention may be formed by the directphotochemical dissociation of such compounds as hydrogen sulfide,dimethyl sulfide, carbon disulfide, methyl mercaptan or of similarmaterials which are capable of direct dissociation in the gas phase toproduce atomic sulfur by a photochemical process as described herein. Aspreviously discussed with regard to the first process embodiment of thepresent invention, some sulfur-containing molecular fragments may beproduced by direct photo-dissociation of the above-noted compounds andmay contribute to the formation of the sulfide layer. However, thepreferred embodiment of the present invention comprises the formation ofdiscrete atomic sulfur which reacts with the vapor phase reactant toform the desired sulfide.

By the above-described process in accordance with this third embodimentof the present invention, sulfur atoms are generated by a photochemicaldissociation process which produces only neutral particles. Thus, thepreviously discussed prior art problems caused by the generation ofcharged particles and high energy radiation and their bombardment of thesubstrate have been avoided. The advantages of this third processembodiment of the present invention are the same as those discussed inrelation to the first process embodiment previously described. Inaddition, the process according to this third embodiment has theadvantage that no photosensitizing mercury is necessary, and thuspossible mercury contamination of the deposited sulfide can be avoided.Further, the apparatus for carrying out the process according to thisthird embodiment is less complex than an apparatus requiring the use ofmercury.

Using this third process embodiment of the present invention, there maybe deposited a layer of any of the sulfide materials discussed abovewith respect to the first process embodiment of this invention, usingthe appropriate selected vapor phase reactant.

Furthermore, in accordance with a fourth process embodiment of thepresent invention, there may be deposited on a chosen substrate asulfide layer which incorporates a selected dopant material by a processwhich uses the direct photochemical generation of atomic sulfur. Theapparatus shown in the FIGURE is used except that the chamber 36 holdingthe mercury is omitted. The process described above with respect to thethird embodiment of this invention is followed except that threereactant gas chambers (e.g., chambers 26, 27, and 28) are used, asdescribed with respect to the second process embodiment of the presentinvention. To accomplish this fourth process embodiment of the presentinvention, the selected vapor phase reactant, such as Zn(CH₃)₂ isexposed to photochemically generated neutral sulfur atoms in thepresence of a dopant-containing precursor, such as tetramethyl tin[Sn(CH₃)₄ ]. The neutral atomic sulfur is produced by directdissociation of a sulfur-containing precursor, such as COS, by radiationof a selected wavelength as shown in Equation (13) below. The atomicsulfur then simultaneously reacts with the Zn(CH₃)₂ and the dopantSn(CH₃)₄ to form the desired tin-doped zinc sulfide, as shown inEquation (14) below. The by-products noted in Equation (14) arehydrocarbon volatiles.

    COS+hν(1849 Å)→S+CO                          (13)

    Zn(CH.sub.3).sub.2 +Sn(CH.sub.3).sub.4 +S→Sn:ZnS+By-products. (14)

A possible alternative to the above-described sequence involves thedirect dissociation of the dopant containing precursor, such astetramethyl tin, to form a neutral trimethyl tin radical as shown inEquation (15) below. The trimethyl tin radical so formed and the atomicsulfur formed in accordance with the direct dissociation reaction ofEquation (13) above then react with the Zn(CH₃)₂ reactant to form thedesired tin-doped zinc sulfide as shown in Equation (16) below.

    Sn(CH.sub.3).sub.4 +hν(1849 Å)→·Sn(CH.sub.3).sub.3 +·CH.sub.3

    Zn(CH.sub.3).sub.2 +·Sn(CH.sub.3).sub.3 +S→Sn:ZnS+By-products

The procedure followed to accomplish the process according to the fourthembodiment of this invention is essentially as described above withrespect to the third process embodiment of the present invention, exceptthat additionally a controlled amount of a selected dopant-containingprecursor, such as tetramethyl tin gas, is introduced from the chamber27 and through the valve 31 into the tube 34, where it mixes with thereactant gases from the chambers 26 and 28.

Thus, by this fourth process embodiment of the present invention, adoped sulfide film may be deposited by a low-temperature process whichavoids the generation of charged particles and high energy radiation andtheir bombardment damage to the substrate, and additionally avoids theuse of mercury for photosensitization. The significance of theseadvantages has been discussed above with respect to the third processembodiment of the present invention. Various dopant materials other thantetramethyl tin may be used and various other sulfide materials may bedeposited as discussed above with respect to the second processembodiment of the present invention.

Finally, by a process in accordance with a fifth embodiment of thepresent invention, a native sulfide layer may be formed on the surfaceof a selected substrate. The term "native sulfide" is used herein todesignate a sulfide generated by the conversion of the top surface ofthe substrate (approximately 10 to 100 angstroms) to the correspondingsulfide. This fifth process embodiment may be performed by practicingthe present invention as described above in relation to the thirdprocess embodiment, omitting the vapor phase reactant. Morespecifically, the apparatus shown in the FIGURE is used, with theomission of the chamber 36 and the chambers 27 and 28. The reactionchamber 10, which contains the selected substrate 20, is evacuated bythe vacuum-producing means (not shown) to a predetermined pressure, forexample, 1 to 4 torr (mm. of mercury). The selected sulfur-containingmolecular precursor is placed in the chamber 25 and the valve 30 is setso that the precursor may flow from the reactant chamber 26 through thetube 34 and then into the reaction chamber 10. The reaction chamber 10may be maintained at approximately room temperature (e.g., 30° C.) or athigher temperatures (e.g., 200° C.). The radiation-producing means 16 isactivated to produce the radiation 14 of a selected wavelength requiredto produce the desired photochemical reaction (e.g., 1849 Å for carbonylsulfide). The radiation 14 passes into the reaction chamber 10, where itcauses the direct dissociation of the selected sulfur-containingprecursor, such as COS, into atomic sulfur, which then reacts with thesurface of the substrate 20 of a selected semiconductor material to formthe native sulfide thereof.

The atomic sulfur required for the process of the present invention maybe formed from the selected sulfur-containing precursor by the discreteabsorption of photonic energy, as previously discussed in relation toEquation (11), for example. The atomic sulfur so formed then reacts withthe surface atoms of the substrate, such as mercury cadmium telluride(HgCdTe), to form the native sulfide thereof comprising (mercurycadmium) sulfide and tellurium sulfide, in accordance with Equation (17)below.

    HgCdTe+S→(Hg,Cd)S+TeS                               (17)

Alternatively, the atomic sulfur required in this fifth processembodiment of the present invention may be generated by the mercurysensitized photochemical dissociation of a selected sulfur-containingprecursor, such as carbonyl sulfide, as discussed herein in relation toEquations (1) and (2) and the first process embodiment of the presentinvention. However, it is anticipated that in this alternative processembodiment, mercury sulfide (HgS) will be deposited during the nativesulfide formation and may cause contamination of the latter.

By the above-described fifth process embodiment of the presentinvention, sulfur atoms are produced by a photochemical process whichgenerates only neutral particles. Thus, the prior art problem associatedwith the generation of charged particles and high energy radiation whichcause damage to the substrate as previously discussed herein has beeneliminated. The incorporation of fixed or mobile charges in the nativesulfide layer formed is minimized by the process of the presentinvention. In addition, an insulating layer may be formed on the surfaceof the native sulfide layer to provide, for example, a semiconductordevice. Such a native sulfide layer enhances the interface propertiesbetween the insulating layer and the substrate. Moreover, by theabove-described process for forming a native sulfide layer, there isalso achieved minimization of the value of the surface state density atthe interface of the semiconductor substrate with the native sulfide andan insulating layer formed thereon. This combination of the nativesulfide layer and the overlying insulating layer is referred to hereinas the "native sulfide/insulator composite." Because of theabove-described effects, the device performance in enhanced by forming anative sulfide layer in accordance with the present invention. Moreover,the above-described fifth process embodiment of the present inventioncan be performed at a low temperature (e.g,, 30° to 200° C.) so thatthermal damage to the substrate is avoided, as previously discussed.

Using the above-described process, the present invention may be used toform the native sulfide of any semiconductor material which is known toform a native sulfide. The process of the present invention isparticularly useful for forming the native sulfides of compoundsemiconductor materials whose constituent elements are known to form thecorresponding sulfide compounds. Such compound semiconductor materialsinclude: mercury cadmium telluride (HgCdTe), gallium aluminum arsenide(GaAlAs), indium gallium arsenide (InGaAs), indium antimonide (InSb),gallium arsenide (GaAs), and gallium antimonide (GaSb). In addition, theprocess of the present invention may be used to form the native sulfideof certain elemental semiconductor materials, such as germanium.

Further, prior to the formation of the native sulfide in accordance withthe process of the present invention, it may be advantageous to use acleaning process, such as a wet chemical etching process as is known inthe art, to provide a clean substrate surface for the formation of thenative sulfide layer.

After the native sulfide layer has been formed by the process of thepresent invention, a chosen dielectric passivation layer may be formedthereon to provide a device having enhanced semiconductor/insulatorinterface properties as described above. Optionally, the substrate mayhave regions of predetermined conductivity defined therein. The processaccording to this fifth embodiment of the present invention isparticularly useful when followed by one of the alternative processembodiments described herein for depositing a sulfide layer, which isformed on the surface of the native sulfide layer. By such a combinationof native sulfide growth and dielectric sulfide layer deposition in acontinuous mode in which the vacuum in the reaction chamber isundisturbed and the substrate is not exposed to the atmosphere betweensuccessive process steps, recontamination of atmospherically sensitiveprepared compound semiconductive surfaces may be prevented. In addition,by using neutral sulfur atoms to form the dielectric sulfide layer, thesurface of the native sulfide can be maintained intact and damagethereto by charge or radiation bombardment can be avoided. The resultingdevice has optimized electrical properties at the interface of thesubstrate with the native sulfide/insulator composite. In particular,structures having these native sulfide layers and dielectric sulfidepassivation layers may be used in the fabrication of HgCdTephotoconductive and photovoltaic devices, light-emitting diodes, andheterojunction lasers, and InSb infrared detectors. A typical structuremight comprise, for example, a substrate of HgCdTe, a native sulfidelayer having a thickness of 20 to 50 angstroms, and a dielectric zincsulfide layer having a thickness of approximately 1000 angstroms.

EXAMPLE 1

This example illustrates the use of the process according to the firstembodiment of the present invention as previously described herein.

Using the apparatus described and illustrated in relation to the FIGUREwith two reactant gas chambers, a layer of ZnS was deposited on thesurface of a wafer of silicon having a two-inch (5.08 centimeter)diameter. Carbonyl sulfide was used as the sulfur-containing precursorand dimethyl zinc, Zn(CH₃)₂, was the selected vapor phase reactant. Anitrogen carrier gas was used to carry the organometallic zinc compound,which has a low vapor pressure, into the reaction chamber 10. Thereaction chamber 10 was evacuated by the vacuum-producing means to apressure of 10⁻³ torr (mm. of mercury), then back-filled with nitrogen,and again evacuated to a pressure of 10⁻³ torr (mm. of mercury) in orderto purge the system of residual air and water vapor. The flowmeters 30and 32 were activated to admit the reactant gases in a predeterminedratio into the tube 34 and subsequently into the chamber 36 and thereaction chamber 10, and the reactant gas flow rates were stabilized.The operating pressure within the reaction chamber 10 was adjusted bymeans of the valve 24 to achieve a pressure of approximately 1 torr (mm.of mercury). The heating elements 21 and the cooling means 25 wereactivated. Finally, the low pressure mercury arc resonance lamps wereactivated and emitted radiation at 2537 Å, which was absorbed by themercury vapor in the reaction chamber, producing photo-excited mercuryatoms, which collided with the carbonyl sulfide to form atomic sulfur.The atomic sulfur then reacted with the Zn(CH₃)₂ to form ZnS, whichdeposited as a layer on the surface of the substrate.

When using a continuous flow photochemical reactor system at anoperating pressure of 2 torr (mm. of mercury), with reactant gas flowrates of 2 standard cubic centimeters per minute (sccm) of Zn(CH₃)₂, 150sccm of nitrogen, and 30 sccm of COS, films of ZnS were deposited at arate of approximately 1360 angstroms per hour, to thicknesses of up toapproximately 1100 angstroms. The ZnS films were found to be specularand durable, (i.e., scratch-resistant) and strongly adherent to thesubstrate (i.e., passed the known "tape test" for adhesion). Inaddition, this ZnS film had excellent morphology, being dense,non-granular and smooth. The refractive index of this material wasdetermined by ellipsometry to be within the range of 2.1 to 2.3, whichis characteristic of ZnS. The chemical composition of the zinc sulfidefilms formed by the above-described process of the present invention wasdetermined by electron spectroscopy for chemical analysis (ESCA) andindicated excellent stoichiometry, i.e., Zn:S=1.0, with minimum oxygencontamination. Films of ZnS are known to be optically transmissive toradiation in the range of 2 to 12 micrometers, which makes ZnS filmsparticularly useful as a passivation or optical coating layer forinfrared detectors, which are active in the far infrared region.

Moreover, using the above-described process, a film of ZnS has beendeposited on each of the following substrate materials: HgCdTe, InSb,GaAs, glass, and quartz. The quality of ZnS film produced on eachsubstrate was substantially the same as that described above.

EXAMPLE 2

In accordance with an alternative embodiment of the present invention, alayer of ZnS was deposited on the surface of a silicon substrate usingthe procedure described in Example 1, except that the sulfur-containingprecursor was hydrogen sulfide (H₂ S). The quality of the ZnS filmproduced was substantially the same as that described in Example 1.

EXAMPLE 3

In accordance with the fifth process embodiment of the present inventionas previously described herein, a native sulfide layer was formed on thesurface of a HgCdTe substrate. The sulfur-containing precursor wascarbonyl sulfide (COS), which was directly dissociated with 1849 Åradiation to form atomic sulfur. Using ESCA and based on the chemicalshifts of the sulfur peaks, it was determined that 24 percent of thesulfur was present in the form of SiO₄ ⁻² and 76 percent was present aselemental sulfur or sulfide (S⁻²).

In addition, using the above-described process, a native sulfide layerwas formed on the surface of a silicon substrate.

While the present invention has been particularly described with respectto the preferred embodiments thereof, it will be recognized by thoseskilled in the art that certain modifications in form and detail may bemade without departing from the spirit and scope of the invention. Inparticular, the scope of the invention is not limited to thephotochemical vapor deposition of zinc sulfide, which was used merely asan example, but is intended to include the photochemical vapordeposition of any chosen sulfide compound from a selected vapor phasereactant that is capable of reacting with atomic sulfur to form asulfide compound. In addition, the chemically unreactivesulfur-containing precursor is not limited to carbonyl sulfide, but isintended to include any material which will photochemically dissociatein the gas phase to produce atomic sulfur, either with or withoutmercury sensitization. Further, the scope of the present invention isnot limited to the use of mercury as the photosensitizing agent, but isintended to include other known photosensitizing and energy transferagents, such as cadmium or zinc, and the use of the correspondingwavelength of the resonance line thereof to produce excitation of thesephotosensitizers. In addition, the process of the present invention isnot limited to the specific operating conditions described herein, whichwere provided merely as examples. In particular, the operating pressuremay have a value higher or lower than the pressure specificallydescribed herein.

Moreover, the scope of the present invention is not limited to thephotochemical generation of atomic sulfur, which was used merely as anexample, but is intended to include the use of any neutral, charge-freeatomic sulfur which is generated in a manner that avoids the formationof charged particles or radiation which may damage the substrate.

Further, the substrate on which sulfide deposition may be performed bythe process of the present invention is not limited to a silicon wafer,which was used herein as an example, but may include other semiconductorsubstrate materials (such as GaAs, HgCdTe, or InSb), electro-opticalelements or components (such as lenses or mirrors, formed of a glass orplastic), or plastic materials. In addition, the scope of the presentinvention is not limited to the formation of a native sulfide layer ofmercury cadmium telluride, which was used merely as an example, but isintended to include the formation of native sulfide layers of binary andternary compound semiconductor materials, such as GaAlAs, InGaAs, InSb,GaAs, and GaSb, as well as elemental semiconductor materials, such asGe, and any substrate material capable of undergoing native sulfidegrowth by means of atomic sulfur. Moreover, the process of the presentinvention is not limited to films of deposited sulfides or nativesulfides used for the purposes described herein, but includes thedeposition or formation of layers, films, or patterns of sulfides forany purpose.

Finally, the process of the present invention is not limited to theparticular apparatus described herein, which was used merely as anexample, but is intended to include any apparatus suitable forconducting a photochemical reaction of the type described herein. Thereaction chamber described herein may have any configuration in which atleast a portion thereof is formed of quartz or other material which istransmissive to the selected wavelength of radiation and may comprise,for example, a hollow quartz tube. Further, the process of the presentinvention may be accomplished in an apparatus in which the source ofselected radiation is contained within the reaction chamber and atransmissive window is not needed.

What is claimed is:
 1. A process for depositing a sulfide layer of achosen element on the surface of a selected substrate which comprises:exposing said surface to a selected vapor phase reactant containing saidelement in the presence of neutral, charge-free sulfur atoms to producea chemical reaction between said sulfur atoms and said reactant in amanner sufficient to form said sulfide layer on said surface of saidsubstrate.
 2. The process set forth in claim 1 wherein said neutral,charge-free sulfur atoms are formed by the mercury photosensitizeddissociation of a chosen chemically unreactive sulfur-containingprecursor.
 3. The process set forth in claim 1 wherein said neutral,charge-free sulfur atoms are formed by exposing a chosen chemicallyunreactive sulfur-containing precursor to radiation of a selectedwavelength to cause the direct dissociation of said precursor to formsaid sulfur atoms.
 4. The process set forth in claim 2 or 3 wherein saidchemically unreactive sulfur-containing precursor is selected from thegroup consisting of carbonyl sulfide, hydrogen sulfide, dialkyl sulfide,carbon disulfide, and methyl mercaptan.
 5. The process set forth inclaim 2 wherein:(a) said vapor phase reactant is dimethyl zinc; (b) saidsulfur-containing precursor is carbonyl sulfide; and (c) said sulfide iszinc sulfide (ZnS).
 6. The process set forth in claim 1 wherein saidvapor phase reactant contains an element selected from the groupconsisting of zinc, indium, arsenic, lead, and cadmium.
 7. The processset forth in claim 1 wherein said selected vapor phase reactant ischosen from the group consisting of dimethyl zinc, tetramethyl lead,arsine, trimethyl indium, and dimethyl cadmium.
 8. The process set forthin claim 1 wherein the temperature of said reacting is sufficiently lowso as to avoid thermal damage to said substrate.
 9. The process setforth in claim 8 wherein said reacting occurs at a temperature in therange of about 30° C. to 300° C.
 10. The process set forth in claim 1wherein said exposing further includes being performed in the presenceof a chosen dopant-containing material and said sulfide layerincorporates atoms of said dopant.
 11. The process set forth in claim 1wherein said substrate is selected from the group consisting of asemiconductor material, a glass material, and a plastic material. 12.The process set forth in claim 11 wherein said substrate has formed onthe surface thereof a layer of a second chosen material, and saidsulfide layer is formed on the surface of said second chosen material.13. The process set forth in claim 1 wherein said layer is formed to athickness within the range of 500 to 5000 angstroms.
 14. A process fordepositing a sulfide layer of a chosen element on the surface of aselected substrate comprising exposing said surface to a chosen vaporphase reactant containing said element and a chosen sulfur-containingprecursor in the presence of radiation of a selected wavelength in amanner sufficient to bring about a chemical reaction to form saidsulfide layer on said substrate.
 15. The process set forth in claim 14wherein said exposing further includes the presence of a chosenphotosensitizing agent.
 16. A low-temperature, charge-free process fordepositing a layer of sulfide of a chosen element on the surface of aselected substrate while simultaneously avoiding damage to saidsubstrate comprising exposing said substrate to a selected vapor phasereactant containing said element in the presence of neutral, charge-freesulfur atoms formed in a manner which avoids the generation of chargedparticles or high energy radiation, to bring about the reaction of saidsulfur atoms with said vapor phase reactant to form said sulfide whichdeposits as said layer on said substrate, while simultaneously avoidingsaid damage to said substrate due to said charged particles or said highenergy radiation.
 17. A process for forming a layer of a native sulfideof a chosen material on the surface of a substrate of said chosenmaterial comprising exposing said surface of said substrate to neutral,charge-free sulfur atoms to bring about a reaction between said sulfuratoms and said surface of said substrate to form said native sulfide.18. The process set forth in claim 17 wherein said neutral, charge-freesulfur atoms are formed by exposing a chosen chemically unreactivesulfur-containing precursor to radiation of a selected wavelength tocause the direct dissociation of said precursor to form said sulfuratoms.
 19. The process set forth in claim 17 wherein said neutral,charge-free sulfur atoms are formed by the mercury photosensitizeddissociation of a chosen chemically unreactive sulfur-containingprecursor.
 20. The process set forth in claim 18 or 19 wherein saidchemically unreactive sulfur-containing precursor is selected from thegroup consisting of: carbonyl sulfide, hydrogen sulfide, dialkylsulfide, carbon disulfide, and methyl mercaptan.
 21. The process setforth in claim 17 wherein said chosen material is selected from thegroup consisting of mercury cadmium telluride, gallium arsenide, andindium antimonide.
 22. The process set forth in claim 17 wherein thetemperature of said reaction is sufficiently low so as to avoid thermaldamage to said substrate.
 23. The process set forth in claim 18wherein:(a) said chosen material is mercury cadmium telluride; (b) saidchosen sulfur-containing precursor is carbonyl sulfide; and (c) saidnative sulfide comprises (mercury, cadmium) sulfide and telluriumsulfide.
 24. A process for forming a native sulfide on a substrate of achosen semiconductor material which comprises:(a) providing a selectedsulfur-containing precursor; (b) irradiating said precursor withradiation of a predetermined wavelength and sufficient to dissociateneutral sulfur atoms therefrom; and then (c) reacting said sulfur atomswith said substrate to form said native sulfide.
 25. A process forforming a layer of a native sulfide of a chosen material on the surfaceof a substrate of said chosen material comprising exposing said surfaceof said substrate to a chosen sulfur-containing precursor in thepresence of radiation of a selected wavelength in a manner sufficient tobring about a chemical reaction to form said native sulfide layer onsaid substrate.
 26. A process for forming a semiconductor device havingoptimized electrical interface properties, comprising the steps of:(a)providing a substrate of a chosen semiconductor material; (b) generatingneutral sulfur atoms; (c) exposing said substrate to said neutral sulfuratoms to cause a reaction between said neutral sulfur atoms and thesurface of said substrate to form a first layer of a native sulfide ofsaid chosen semiconductor material, said native sulfide being formed ina manner which eliminates damage to said substrate due to exposure tocharged species or high energy radiation; and (d) forming a second layerof a chosen dielectric passivation material on the surface of said firstlayer of said native sulfide, to form a native sulfide/insulatorcomposite, whereby the interface of said substrate with said nativesulfide/insulator composite has said optimized electrical interfaceproperties.